Anecdotally, it seems that few horses keep their form/class at age 8 or up.
So, either assuming the assumption correct or adjusting age, once you calculate ex-age, ex-residual value value par (however you choose to do so), what discount to par do you demand per year?
10%?
25%?
Alternatively, is there an age where, irregardless of to-date success, you just say no and move on?